Frequency shift receiver



May 16, 1950 J. c. E. MITCHELL FREQUENCY SHIFT RECEIVER 5 Sheets-Sheet 1 Filed May 16 1946 INVENTOR J. C. E. TCHELL BY ATTORNEY May 16, 195o .J.C.E.MHCHELL FREQUENCY SHIFT RECEIVR Filed May 16, 1946 5 Sheets-Sheet 2 gay. 2a

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May 16, 1950 J. c. E. MITCHELL.

FREQUENCY SHIFT RECEIVER Filed May 16,l 10946 c 4 @.500 g L s OSC/MIUR [UCK-IN Q CHARACTER/sT/c- L.. LL. 3 s/NGLE sfo/VAL gi. Lu LL Q) LL 5A WAT/0N :i 6 7' 5 3000 Q 4 k V) V) i500 Lu 3 w Q Q 2 E f g b 0 .25.5.75/ 1.25 APPL/ED VOLTAGE CYCLES DEV/,4770Al FROM FREE OSC/LLA/O/V FREQ. 0F LCKED-/N FREQUENCY 10g. m

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FREQUENCY SHIFT RECEIVER Filed May 16, 1946 5 sheets-sheet 4 OR/G//VAL KFYED SIGNAL X lag 6b i275 /Y 350 1F +425 cps o 1275 (0a/*Pur FROM M1/TER) Elige 2 Es 1F I+ cps LJ 0 s 50 2 Q our/0r FROM 0c/(E0 @5e/mm7 b. Q (Lock-w RANGE .gf/anw 00H50) Q- Q INVENTOR J. C'LZVHTCHELL BY ATTORNEY 5 Sheets-Sheet 5 J. C. E. MITCHELL FREQUENCY SHIFT RECEIVER May i6, 1950 Filed May 16, 1946 INVENToR J C. E. BY

ATTORNEY IHELL atentecl May i6,

FREQUENCY SHIFT RECEIVER John C. E. Mitchell, Montreal, Quebec', Canada, assignor to Radio Corporation of America, a

corporation of Delaware Application May 16, 194'6, serial No. 670,035

(ci. 25o- 8) Claims.

This application discloses an improved receiver for frequency shift telegraphy, facsimile and similar signals.

In order to reduce signal distortion, signal mutilation and signal dropout resulting from multipath transmission, etc., on telegraphy, facsimile and similar signals, frequency shift systems known in the prior art usually make use of multiple receivers in space diversity systems and in some cases of multiple receivers in frequency diversity as well as space diversity. See for example Lyons U. S. application Serial #578,877, led February 20, 1945, and Lyons U. S. application Serial #648,965, led February 20, 1946.

These known systems are quite complicated and are of considerable original cost and quite expensive to keep in operation. The primary object of my invention is to provide a receiver of frequency shift signals which is simple in nature and of relatively low cost and yet provides an output for recording or similar purposes which is sufficiently free of signal distortion, signal mutilation and signal dropout to be relied on for communication purposes.

Briefly I attain this object by provision of a. new and improved tunable receiver comprising selective circuits of suiiicient band Width to permit local oscillator or transmitter drift within reasonable limits, thus rendering unnecessary automatic frequency control circuits. The improved receiver includes as a novel combination, an amplifying current amplitude limiter which improves the signal to noise ratio and increases the signal strength with an oscillator locked in by the limiting amplifier which oscillator repeats and further amplifies and shapes the signal pulses to provide for detection purposes full signal output when signal input of reasonable strength and quality is received. The novel combination also provides means for using signal where mark or space is mutilated and includes with the amplifying limiter locked oscillator, a frequency discriminator and detector having a characteristic of sufficient width to provide linear output over the frequency shift range.

The detector output controls a rst locking circuit which in turn controls a locking circuit such as described in Cohoon U. S. application Serial No. 649,343, filed February 21, 1946, nowl abandoned.

The receiver of the present invention is to be used for reception of frequency shift signals which terminate in a given condition, for eX- ample, in mark condition, and is to supply said signals to recording apparatus such as a .tele-Y typewriter arranged to start operation on the reception of space. The locking circuits have two conditions of stability and in practice may not be left in mark condition when signaling is terminated, or, when no carrier signal is being received the locking circuits may be tripped to space condition. A further object of my invention is to provide an improved circuit arrangement for tripping the locking circuits to the mark signal condition in the absence of a received carrier signal if the system is not already in such condition. Briey this object is accomplished by utilizing a potential developed in the current amplitude limiter circuit which is substantially linearly related to the received carrier strength for producing in the locking circuit the same result as would the appearance of a mark signal. This means is not to be confused with the trigger or locking circuit restoring means of my U. S. application Serial No. 659,477, filed April 4, 1946. The Locking circuit restoring means operates to return the locking circuit to mark condition if tripped to space condition by a noise pulse. The present means maintains the locking circuit in mark condition when there is no signal carrier being received.

The manner in which the above objects and other objects the nature of which will appear -in detail hereinafter are attained will now be described in detail. In this description reference will be made to the attached drawings wherein Fig. l illustrates schematically and by block diagram a frequency shift receiver such as, for example, one receiver of the diversity systems referred to hereinbefore. Applicant makes no claim to the receiver of Fig. l.

Figs. 2a, 2b and 2c are curves or voltage diagrams illustrating operation of the arrangement of Fig. 1 and also illustrating the manner in which multipath transmission 0r fading or noise affects the signal in such a receiver thereby illustrating the need of my invention.

Fig. 3 illustrates schematically and by block diagram a frequency shift telegraphy or facsimile receiver arranged in accordance with my invention.

Fig. 4 illustrates details of the current limiting amplifier' and locked oscillator of the arrange ment of Fig. 3.

Figs. 5a and 5b illustrate by curves the operation of the limiting amplifier locked-in oscillator.

Figs. 6a, 6b, 6c and 6d are voltage diagrams illustrating the manner in which noise is reduced by the use of my limiting amplifier locked-in oscillator.

Figs. 7a, '7b and 7c illustrate by voltage diagram 3 the output derived from the limiting amplier locked-in oscillator and locking circuit.

Fig. 8 illustrates by circuit element and circuit element connections a receiver arranged in accordance with my invention and including the novel combination of the current amplitude limiter locked-in oscillator, linear detector and the locking circuits controlled thereby with my improved squelch circuit actuated by the potential drop in the amplifying limiter for keeping the locking circuit in the mark condition in the absence of received signal.

Known frequency shift receivers used singly or in space diversity and/or frequency diversity may be as illustrated in Fig. 1 'andl mayincludfe the following features. A means of signal selectivity such as found in the tuned circuits of a radio frequency amplifier 2, and in particularin the tuned circuits of the intermediate frequency amplier 4 of a conventional amplitude modulation receiver, followed by a conventional limiter I 0, and a 'discriminating network I3 and deitector which is a modification lof the discrirnf inator and detector illustrated in Seeley Patent #2,121,103. A large capacitor I2 couples the disfcriminator output to the input of a signal ampli'f fier I4. The purpose of this large output coupling capacitor I2 has been described in detail in the abandoned Peterson'U. S. vapplication Se? rial #630,428, filed November 7, 1945, and is to permit signal drift from the center frequency without changing the polarity or sense of polarity of the signal pulses. The'signal pulses are then lteredin unit I6 and actuate double locking Vcircuits I8 and 2i) of the'type Ydescribed Yherein and in the abandoned Cohoon U. S. application Serial #649,343, led February 21, 1946, Vor in Schock et al. U. S. application Serial #632,978, led December 5, 1945. A large'condenser I'I having the same purpose as the condenser IKZ is inthe coupling between units IB and I8. The locking circuits control a teletypewriter, facsimile Yrecorder orsimilar recording device. Y

The practical operation of this system requires the following operating conditions:

a. In the absence of automatic frequency control means selectivity must be such that the pass band is suciently wide to allow for local os-f cillator drift in the receiver and oscillator drift in the transmitter. This means that the eiective bandwidth must be greater than that required to pass the two signal frequencies (mark and space) and their side bands. (Usually two or three times as wide.)

b. Adequate amplification mustbe used ahead and thereby masks lack of limiting on the weaker 6 frequency. Slow AVC is also a disadvantage in this respect and in most receivers that I have come across the AVC cannot be used because itestablishes the gain of the receiver on the strength of the stronger frequency. These points are relatively unimportant in the diversity sys-f tems because the diversity systems are arranged to use only the stronger instantaneous frequency whereas in the single receiver the weaker frequency must be used in conjunction with the' stronger frequency.

c. That such selectivity be provided before nal limiting.

d. The linear portion of the center of the discriminator curve must be as wide as that required in (a).

e. The insertion of a large coupling condenser I2 between .the discriminator output and the input to the locking circuit to allow the signal to `drift from the center of the discriminator curve and still retain its sense or polarity.

f. That the coupling condenser I'I in conjunction with the grid ,resistor I9 and the input grid of the locking lcircuit in unit I8 act as a direct current restoring device for detected (discriminator) signal voltage. The major advantages of this system are:

1. Drift of the intermediate frequency signal may be accommodated within the pass band of the receiving system.

2. The system operates on transient voltages from the discriminator and consequently it' is not necessary that the whole area of the received signal be present but only that the potential difference between successive signal characters exist.

The system illustrated Fig. 1 and similar systems beve Certain disadvantages- .Iptei'ferine siede! .er noise remeved frein the Wanted desde by @in apPrG-Cablfequey @'fern' 'eue'able .59, Create @n apprlble number .0f .elrOfSr ThS is dde in eert et leeette the, use ef Wide bend pass circuits necessary the absence of auto: matic frequency control to allow for carrier drift'. The manner in which ythe errors are created is illustrated in Figs. 2a, 2b and'2c. In Fig. A21a the cutput of a discriminator is represented and iniudes as Well as the wanted signal three points at which the interfering signal has saturated the limiter and produced unwanted output. These peints are ehewd et A, B end C. Fig. 2b 1111.15- trates the signal as originally transmitted before the same has been subjected to interference andr the noise resulting therefrom. Fig. 2c represents the resultant output from the system which might. be supplied vfrom the. locking circuit unit 20 and includes theerrors introduced by interference or noise. These errors are shown by shaded lines in Fig. 2,0.

An object. of the present invention 11S te nrevidc a single simple Areceiver for frequency shift and similar signals wherein the effects of inter- .ference of the nature described above are sube stantially reduced and so ,effectivelyneliminated that good results are obtained by use of the same,

A second object of the present invention is to previde e simple and relatively inexpensive re-l eeiveli for frequency. Shift and .Similar signals wherein the effects of interference of the nature described ebeve elle. reddeed and Whieh receiver ddee. dei rely en. Space diversityv eleets te reduee the interference: 13203.15@ tellgelc@ is tldllmitted o n two frequencies (iFS) a vform of free quency diversity is involvedv if one considers that only one of the two frequencies is quite often all,that is required to produce a satisfactory out-v put. character.-V

An additional object of the present invention is to provide a simple and relatively inexpensive recever for frequency shift and similar signals wherein the effects of interference of the nar, ture described above are reduced and which re-4 ceiver does not rely on the use of automatic fre,- quency control connections for tuning purposes.

A more detailed object of the present invention is to provide a system for frequency shift telegraphy and similar signals comprising a single receiver in a simple circuit having in general the makeup of the prior circuit of Fig. 1 but including means for eliminating the disadvantages mentioned above and in particular output distortion caused by interfering signals or noise resulting from multipath transmission. An embodiment of my receiver for attaining this 4object is illustrated in Fig. 3. In Fig. 3 by an improved combination of means, reduction of the effects of interference is accomplished and the novel combination of means does not unduly add to or complicate the receiver system. Moreover it allows reception of signals under interference conditions which would render it impossible to receive signals on known receivers which do not make use of diversity effects, and yet it retains the advantages of a band pass two to three times the width of the frequency shift in accordance with signals. My novel receiver then has all of the desirable features enumerated above in the discussion of Fig. 1 and none of the undesirable features, and yet does not involve automatic frequency control circuits or receivers in diversity.

The receiver of my invention is essentially thel same as that described and illustrated in Fig. l except for the provision of more gain in the stages preceding the limiter and the introduction of a locked oscillator II, interposed between the current amplitude limiter IU and the frequency discriminator and detector I3, as illustrated in Figs. 3 and 4 of the drawings. Any current amplifying limiter having the desired characteristics may be used but preferably I make use of an arrangement as ilustrated in Fig. 4 and Fig. 8 of the drawings.

The frequency shifted signal is picked up and amplified and converted to a lower frequency in units 2 and 4, Fig. 3. The output of unit 2 which may be at any intermediate frequency IFFS, where FS is the frequency shift, is fed to the first stage of an IF amplifier 4, the last stage of which may include tube VI Fig. 4 coupled in a tuned amplifier circuit. This circuit includes an input transformer (not shown) tuned to pass a band of frequencies considerably greater than IFiFS. The anode of tube VI is connected to an inductance LI and condenser CI parallel tuned to the frequency IF. The gain of the stages preceding the amplitude limiter V2 is stepped up until limiting takes place on mark and space frequencies irrespective of their relative strengths. The current amplitude limiter and locked-in oscillator comprises a limiter tube V2 having its control grid GL coupled by a resistor RI and coupling condenser C2 to the tuned anode circuit CI, LI of the intermediate frequency amplifier. The anode AL of the limiter tube V2 is connected with a potentiometer resistor R2 and by a second resistance R3 to the positive terminal of a source B3 of direct current potential. Potential for the screen grid of the limiter is supplied through a potential dropping condenser R1. The tap point on the potentiometer R2 is coupled by .condenser C over a grid biasing resistor R4 to the control electrode GO of a locked in oscillator including tube V3. The tube V3 has its screening electrode and a second control grid regeneratively coupled by transformer T2 including inductance L2 coupled to the parallel tuned circuit comprising inductance L3 and condenser CI. Direct current potential for the screen grid is supplied through the inductance L3 While biasing potential for the third grid is supplied by resistor R5. The grid alternating current circuit includes a coupling condenser C6 and a resistor R6. The' anode AO of the tube V3 is coupled to the primary winding L4 of a transformer T3 in a modified Seeley discriminator and detector system. The primary inductance L4 is in parallel with tuning condenser C8. The anode AO is also substantially directly coupled to the diodes in parallel by a separate lead 30 running from the anode to the tuning and direct current blocking condensers CIU and CII.

The operation of the system is as follows. Voltage from the intermediate frequency amplifier which may be of the order of 400 kc. developed across the circuit LI, CI which is parallel resonant to the said intermediate frequency. This intermediate frequency voltage is fed through the coupling circuits C2, RI to the control grid GL of the limiter tube V2. Capacitor C2 and resistor RI comprise a short time constant grid input circuit (say of the order of 10 to 18 microseconds), and this in combination with a lower than normal screen voltage supplied through the resistor R'I and in cooperation wtih the IF bypass condenser C3 insures operation of the tube V2 as a conventional current amplitude limiter. In this limiter the tube V2 is biased beyond cutoi by negative excursions of the alternating current, thereby limiting the peak amplitude of the currents in the negative direction. Due to the low screen grid potential supplied by resistor R1 and the relation thereto of the magnitude of the driving currents the tube is driven beyond saturation on the positive cycles to thereby accomplish current peak amplitude limiting on the positive peaks.

Voltage at the intermediate frequency which has been limited appears across the potentiometer R2, this voltage being of a constant alternating current magnitude for varying amplitudes of intermediate frequency signal applied to the input of tube V2, provided the signal is greater than a selected critical value, say 3 or 4 volts at the grid of the limiter. That is, limiting begins ,to take place with this arrangement of tube and components at 3 to 4 R. M. S. IF signal volts on the input grid of V2. Voltage from the potentiometer resistance R2 is coupled through coupling and blocking condenser C5 and grid leak resistor R4 to the control grid G0 of the locked oscillator including tube V3. The capacitor C5 and resistor R4 also provide a short time grid input circuit in so far as the characteristics olf the tube V3 as a second limiter are concerned. Capacitor C4 and resistor R3 provide a lter network for the plate supply connections.

The tube V3 is an entrained or locked in oscillator. Its free oscillation frequency is determined by the tuned tank circuit comprising in- `ductance L3 and capacitor C1, this circuit being tuned to the center of the intermediate frequency band. The locked oscillator tube may be of the heptode type wherein the second and fourth grids, i. e., the screen grid, serves as the plate of the oscillator, and the #3 grid serves as the oscillator control grid. Feedback voltage is obtained from inductan-ce L3 through inductance L2 coupled thereto and fed to the #3 grid through coupling capacitor C6. The resistor R6 serves to prevent resonance in L2, and R5 is the oscillator grid bias resistor. The locking or control signal is fed to the first grid GO from the anode of tube V2. Voltagev of a frequency near the free or unen- .trained oscillator frequency fed to the grid` GQ doesfnot varyfthe amplitude-of the oscillations generated but serves to vary and control their frequency. The amount by which thefrequency of the oscillator may be varied is determined by the amplitudeof the signal applied to the .grid GO, that is to say, the variation in frequency inherent in the received signal may vary the `oscillator frequency to an extent proportional 'tothe amplitude of the received signal fed from Athe limiter 'tube V2. In a practical embodiment .5 volt I. F. fed to the' oscillator V3 held it entrained or in locked condition overalfrequency swing of 3000 cycles, while approximately .25 I. F. volt alternating current Willhold-it entrained or locked in for only i500 cycles. Obviously in either case the oscillator is entrained over a band of frequencies-sufficient for use in a frequency shift signalling system Where the frequency shift FS may be of the order of -several hundred to several thousand cycles. Of course theamplitude of the entraining signal supplied to the grid GO 'of theltube V3 is constant for a given setting of the potentiometer point on R2, hence R2 serves as a control on the locked-in range and consequently .a selectivity control. The output of the locked oscillator which follows the frequency of the .limited frequency shift currents fed to thegrid GO of tube V3 is impressed -on a discriminator` and detector of a modified Seeley type. This discriminator and detector includes a transformer T3having a parallel tuned primary including inductance L4 and capacitor C3 and a `parallel tuned secondary including inductance L6 and capacitors Cl and CII and a pair of diodes -differentially connected by thesecondary winding L6, and also coupled in like phase for .in parallel by the lead 3D. The operation of this discriminator and detector is well'knownirrtheart, and will 'not be described Vfurther at this point. The output may be taken between one cathode and ground and used as desired and as described vhereinafter.

'An important'factor in the'operation ofsuch a limiter-locked oscillator circuit is its operation in the presence of random noise voltages. Operating under the conditions described above. i. e.,r'beloW saturation of the control signal applied to -grd GO,`see=Fig.5c, the frequency of the locked oscillator will tend to return to its free oscillation frequency in fthe absence'of na discrete frequency signal voltage. This disengagement of the Voscillator from the infiuence of the controlvoltage, when the same disappears 'or becomes .too low, 'hasrbeen called its breakout characteristic. 'For a signal consisting of a'random .distributioniof signal, and thereby prevents the teletypezchar.- Aacter from being destroyed.

The limiter-lockedroscillator serves toiimprove "reception in the followingmanner. Eigashows a keyed signal which, whentransmittedby'frequency shift methods, received at the loutput ofthe receiver-limiter-asshown in-Fig.-6b. 4Points X, LY andvZrepr-esent the frequency Yof voltages greater than Ythe signal which has saturated the? limiter at these times. The frequency of 'the locked oscillator isshown in Fig. 6c. It will be noted that at-times X, Y and Z the oscillator has returned-tovzero frequency deviation (its free voscillation frequency) because the ycontrol signal' frequency (6b)v Vhas swung beyond Vthe lock-in range (showndot'ted in Fig. 6c). As is shownin Fig. 6d, these interference bursts have no effect on theI signal because a voltage swing of 2/3 to SV; of the peak-to-peak signal voltage (depending on setting of threshold control) is required to trip the locking circuit I8 Fig. 3. It is clear that if the'inter-ference pulse fully saturates the limiter outside the lock-in range of the oscillator there will be practically no lock-in voltage at theirequired lock-in frequencies and the oscillatoriwill' break-out and return to its free oscillating frequency. However, even in the event 'that the interferencepulse is barely greater in amplitude than the signal, thus producing a random 'dls' tribution of frequencies inside the lock-in range, the oscillator will tend to lock to frequencies near est its free oscillation frequency in view of its lock-in sensitivity characteristic (see Fig. 5b). Similarly, at a time when the signal fades into-the noise and mainly a random distribution of noise voltage is left the oscillator will exert aform of selectivity on the noise and limit theswing'of the noise voltages. This characteristic is important when selective fading is present.

Fig. 7a shows a received signal from a keyed waveform'as in Fig. 6a. It will be noted that the mark signal at U and W and the space signal at V have faded into the noise. This may occur during frequency selective fading. The tendency of the oscillator to lock to those frequencies near its free oscillation frequency is shown in Fig. f7 b. Here U, V and W show much smaller frequency excursions. In fact in these particular casesl the conditions are such that the original signal is restored (Fig. 7c) at the voutput from the locking circuit.

`An attempt has been made to show, in the foregoing paragraphs, that a signal may be restored 'even though it has undergone unfavorable propagations'and reception conditions. However, it is important to note that interference and noise of the type shown in Figs. 6b and 7a will practically always destroy the pulse following it as well unless means are employed to reduce the frequency excursion, and consequent detected D. C. voltage change, involved.

It will be noticed that'these advantages stem largely from thebreak-out characteristics of: a. locked-in oscillator. An examination of this characteristic in lock-in oscillators seems to indicate that the best mode of operation occurs when the oscillator is locked Yat its fundamental frequency.

`In Fig. 8 I haveaillustrated the essential features of a frequency shift receiving system arranged in Vaccordancerwith my invention which proved very satisfactory in operation. In this gure I have omitted-the radio frequency amplifiers, converters, etc., and it is assumed that frequency shifted energy of intermediate frequency of about 400 kc. iFS is'impressed on the input of transformer Tl. The limiter including tube V2, the lockedfin oscillator including tube V3 and the discriminator and detector including the diodes V4 operate sub.t stantially as described above to supply keyed pulse energy to the reversing switch RS. The frequency Yshift modulation inthe embodiment being-'de- `scribed may totalv about 850 cycles. Circuits 'T S and T4 are tuned to the intermediate frequency which may be say 400 kc. It is, of course, possible to operate the locked oscillator including circuits T3 and T4 at a sub-harmonic of the intermediate or fundamental frequency, Preferably the oscillator is locked-in at the fundamental because at F/n there are many combination frequencies at which random noise may lock, whereas at F random noise tends to lock very near to the free oscillator frequency.

The circuit including condenser CI4 and inductance LI3 is parallel tuned to the intermediate frequency and serves as an impedance across which the IF voltage is developed by the intermediate frequency amplifier tube VI. Resistor RIB and condenser CIB serve as a plate supply filter. The voltage developed across condenser CI4 and inductance LI3 is impressed by coupling condenser CI5 on the first grid GL of the-limiter tube V2 which as in the prior embodiment operates as a conventional grid plate limiter. C'ondenser CI5 and resistor RI4 provide a short time constant grid input circuit. Condenser CIS` and .resistor RIB provide a lower than normal screen grid voltage thereby establishing the proper operating conditions for the current amplitude limiting action in tube V2 which is as in Fig. 4.

Intermediate frequency voltage limited in tube V2 appears across the resistor R20 and resistor R22 for the broadselectivity position of switch S, and across resistor R20 and resistor R2 I, with resistor R22 in parallel in the sharp selectivity position. The resistor R24 and condenser CIS serve as a plate supply filter. Resistors R20, R22 and R2I serve as a form of potentiometer by which a portion of the IF voltage from tube V2 is fed through condenser C20 to the grid of V3. With switch S in the broad selectivity position that portion of the IF voltage developed across R22 is used, while a smaller portion of the voltage that is developed across the resistors R2I and R22 in parallel is used, when switch S is in the sharp selectivity position. Condenser C20 and resistor R24' provide a short time constant grid input circuit insofar as the operation of the tube V3 as a second limiter is concerned.

selectivity, improved over that which occurs in the known receivers, i. e., which do not have my locked-in oscillator, occurs by virtue of the combination of the current limiting stage including tube V2 and the locked-in oscillator stage including tube V3. Operation of vthe locked-in oscillator is similar in some respects to that described by G. L. Beers in his U. S. Patent #2,356,201, dated August 22, 1944. See also-page '73() of the IRE of December 1944. It is noted, however, that here the oscillation is locked-in at the fundamental frequency rather than at a subharmonic thereof as in Beers. The amplitude of the IF voltage fed from the limiter stage including tube V2 to the locked oscillator tube V3 is just sufficient to provide a locking range of 3000 cycles in the broad selectivity position of switch S, and 1500 cycles in the sharp selectivity position. In the embodiment tested .5 volt 'and .25 volt were used for the broad and sharp selectivity positions respectively. Interference voltages at frequencies outside the locking range and having amplitude sufficient to saturate the limiter caused a break-out characteristic similar to that described by Beers. Advantage is taken here of the fact that such break-out does not generally cause distortion of a teletypewriter signal. This is because the change in voltage during break-out is only one-half the mark to space change developed .by the wanted signal, while two-thirds of the mark to space voltage change is required to trip the first locking circuit LC including tubes VB and V'I.

In the operation of this equipment, for reasons given above, it is important that the IF voltage at the grid of limiter tube V2 due to the weaker of the two frequencies (FiFs) be 5 Volts or greater to insure saturation of the limiter at all times. A grid voltage at tube V2 of 20 to 30 volts (IF noise) when the receiver antenna circuit is short-circuited has been found desirable for best operation. It is evident that tube V2 may also be a locked oscillator limiter if circuit conditions warrant. i

The locked-in circuit is otherwise substantially as illustrated in Fig. 4, the transformer T3 comprising a winding LIQ in the third grid circuit and a winding LI5 between the screen grid of tube V3 and the potential source -I-B3. Voltage dropping resistors R28 and R29 are included in the last connection. A bypass condenser C24- completes the radio frequency circuit. The circuit including condenser C26 and inductance LIS are parallel tuned to the fundamental or IF frequency. This circuit is shunted by biasing and damping resistor R30 and coupled by condenser C29 to the third grid of tube V3. The anode, which is electronically coupled to the oscillation electrodes and circuits, feeds the frequency shifted oscillations to the frequency discriminator network T4, which needs no further explanation.

The purpose of the switch RS is to permit reversal of the output of the diode detectors V4 in the event it is desired to change the relation between mark and space frequencies and the detector output pulse magnitude representing mark and space for any reason. The tuning meter M is also arranged to be connected across the output of the diodes V4 in either position of RS for tuning purposes. The output is fed by a large condenser CI2 (Fig. 3 also), and potentiometer R40 to an amplier 40 and from the amplifier 40 to a keying speed selecting switch KS through the large condenser CII (Fig. 3 also) and resistor R46 to tube V6 in a first locking tube circuit LC. The low pass filter LF may be omitted when the keying speed is high.

The locking tube circuit LC comprises, as is well known in the art, a pair of electron discharge devices V6 and V'I so arranged that when one draws current the other is cut oif and vice versa. In the embodiment illustrated the control potential is applied to the first grid of the ,tube V6 and this tube is non-conductive in marking condition. The tube V'I is conductive in marking condition and non-conductive on space. The discriminator and detector detects the frequency shifts and supplies pulses of direct current to condenser CI'I which vary between mark condition, wherein they are sufficiently negative to cut off current in tube V6, to space condition, wherein they are sufficiently positive to cause current to flow in tube V6.

The anode of the tube V6 is couped to the grid of a tube VII) in a second output locking or tripping circuit LC including another tube VII. The arrangement here is as disclosed in Cohoon U. S. application Serial #649,343, filed February 21, 1946. The current flow through VIO passes through resistance REI so that the grid of tube VI I connected to R5I by resistance R52 becomes more negative when VIO draws current. The

,current through tube VII and resistance R58 11 serves in the same' way to bias the grid of tube VI@ through resistance R1I when VI I draws current.

In the sake of simplicity the locking circuit LC' here is shown in the'polar connection so that the current new iVIE) through'the tube VID is from plus BI through the tube impedance to ground and from ground through the relay connected to the terminals a; e of the receptacle 60, through the lter F, the meter MI, and through resistor R51 to the negative terminal of source BI. Current flow through the tube ViI passes through the typewriter' relay in the opposite direction. This path for the tube VII current 1V II is from plus B2 through the tube V'I I, through the meter MI, the iilterFVthe teletypewriter relay Winding to ground and from ground back to the resistor R58 to the negative terminal of source B2. In this manner theV teletypewriter relay is operated and the signal is recorded. The variable resistance RSI is in shunt to the teletypewrter relay and serves as a means to vary the current therethrough inv polar connection as shown. In the neutral position not shown it is in series with the teletypewriter relay. Resistor R66 is included in the absence of resistancein the external line (connected at a and e) to damp the teletypewriter relay coil. Meter MI measures the teletypewriter current. The lter F reduces' the radiatednoise voltages from steep wave fronts of the output current and also prevents internal pick-nlp of voltage from strong external elds in the vicinity of the equipment.

In operation the potential at the negative end ofv resistor R58 is minus substantially 60 volts when the tube V! I is conductive. This tube conducts on space and this negative potential may be used through lead 69 to operate a tone keyer including tube VI3 and tone source 14. When the tube VI I is conductive currentflows through the resistance R58 to make the end thereof adjacent source -B2 highly negative. This condition takes place on space and the highly negative potential on the third or suppressor grid of tube VIS- makes the same non-conductive so that no tone is supplied through the tube VI3 and tone transformer 50 to the outgoing line plugged into the receptacle at c and d. When the tube VII is cut off as it is during mark no current flows through R58 and the potential at the upper end thereof assumes about groundV potential. The third grid of tube V lbecomes less negative and this tube is adjusted and so excited that it now passes tone to the outgoing line. Thus we have keyed tone output, a pulse of which is present during marking condition and blocked off during spacing condition.

Since in the embodiment described tube V6 is switched to be non-conductive in the mark condition, its anode potential then goes up as does the potential on the grid of tube VII! and tube VIQ is conductive in the mark condition. This turns tube VII off in mark condition.

In the embodiment described the following type tubes are used. VI is a 6SG'1; V2 is a GSJY; V3 isa- GSA'Y; V4 is a (il-I6; VE and V1 are GSJ'Is; and VI@ and VII are GLGGAs.

In the embodiment described the messages start on space and terminate on mark If the signalcarrier drops out or is not being transmitted, i. e., between messages, the locking circuit may be left in the space condition and no tone -output will be supplied.

I provide an improved circuit set in operation in the absence of carrier energy for setting up in oneV ofthe tubes' ofV one of the locking'circuitsthe condition at which said tube would operate on the reception of marking signals. Then the teletypewriter will be receiving mark output current and the tone keyer V|3 will be conductive to supply tone output when no signal is being received. To do this I connect the plate of tube V9 through a resistor R10 to the anode of tube V1 and screen grid of Atube V6. Thus the tubes V1 and V9 have their anodes supplied by direct current potential through the same resistor R14 from the same source. The resistor R10 is a current limiting resistor. As pointed out hereinbefore the tubeVl conducts during reception oi av mark signal. The operation of the tubes V6 and V1 issuch that their anode voltages are approximately 20v volts when the tubes are conducting and 120 volts when the tubes are non-conducting; A reduction in the plate voltage of'one tube causes it to conduct because this reduction serves to'cut off the plate current inthe other tube by lowering its screen grid voltage when bias existing across the cathode resistor R12 is considered. Cutting oir the plate current of the second'tube causes its plate voltage to rise which increases the screen voltage on the iirst tube and allows the rst tube to conduct. This is fundamental in locking circuits ofV this type. Under normal signal conditions transition or ipping of current between the tubes occurs when a voltage is applied to the grid of the rst tube, in this case, tube V6. Hence if the tube V9 is caused to draw plateK current through resistor R14 (at the anode of tubev V1) theV plate voltage of the tube V1 willl be' reduced and V1 will become the conducting tube and the output current of'the equipment' will be on mark, that' is, the tube V1 will draw current as it does during signaling in the mark condition. Tube VII will then be cut off as it is in the mark condition.

The screen grid of tube V9 is xed at approximately volts and is supplied by resistors R18; R19 and'R'BUl If a bias of --6V volts or more is applied to the control grid of tube V9 it is essentially non-conductive and has no eiect on vthe tubes V6' and' V1, that is, it does not lower the potential on the anode of the tube V1. However, if minus 4 volts or less is supplied to the control grid oi4 tube V9 it becomes conductive and draws plate current through resistor R14. This drop in potential at the anode of tube V1 causes the tube to become conductive, in which case it is in the; mark condition to supply output current to the teletypewriter and tone to the keyed line. A bias which is a function of the signal strength isv supplied' to the grid of tube V9 from the grid GL of the current amplitude limiter tube V2. Rectiiied signal current flowing in the grid circuity of tube V2 develops` a voltage, depending on the current intensity, which appears across grid resistor RI4 in the conventional operation of this limiter. This rectified voltage which is negative at' the grid GL is reduced approximately lO'to l by resistors RI5, R81 and R83, and is fed to the grid of tube V9 through an RC filter' including lresistor R84 and condenser C50. Condenser CSI serves as an IF filter. The RC 'filter including resistor R84 and condenser C60 serves to remove unwanted audio components from the bias developed' on the gridV of tube V9 and also to prevent instantaneous high amplitude noise peaks from beingv applied to this grid. The time constantY of this circuit is approximately .2 second. The combination of resistors R15, RBI and R83'also serves to isolate the slow timev constant -circuit R84 and 13 condenser C60 from the grid of tube V2. As the IF signal on the grid of tube V2 is generally much greater than 60 volts peak, and therefore produces a D. C. potential of the same order of magnitude, it can be seen that tube V9 is cut olf and has no effect during normal reception of signals. This is because R83 L Rl5-l-R8l-i-R83ul0 and atleast 6 volts (minus) D. C` is applied to the grid of V9. However, when a wanted carrier is not being received the voltage on the grid of V2 is generally less than 40 volts peak and consequently tube V9 is conducting and the output current from the equipment is mark. Variations in the values of the resistors RIS, RBI and R83 can be made to accommodate different IF voltages than those mentioned. This mark maintaining circuit is necessary in the rabsence of received signals, as otherwise the teletypewriter will print gibberish from the presence of noise impulses generated in the system which operate the locking circuits. The tube V9 is of the SSJ'? type.

What is claimed is:

1. In apparatus for demodulating electrical energy which is shifted in frequency between two frequencies separated by a band of frequencies, to derive direct current voltages which vary between two values as said frequency is shifted, for operating a locking circuit controlling a recorder, said locking circuit including two electron control devices one of which is to be conductive in response to the presence in the apparatus of electrical energy of one frequency and the other of which is to be conductive in response to the presence in the apparatus of electrical energy of the other frequency, a regenerative oscillation generator of variable frequency, an electron control device current amplitude limiter excited at its input by said electrical energy and having an output circuit, a coupling between the output circuit of the current amplitude limiter and the oscillation generator for entraining the generator in accordance with the limiter output, a frequency discriminator and detector having a frequency variation versus voltage characteristic which is linear over a frequency range greater than said rst mentioned band coupled to the output of said generator, and means for applying the output of the detector to said locking circuit to operate the same and control said recorder.

2. In a signaling system in combination, a locking circuit including two electron control devices one of which is to be conductive in one signaling condition, a current amplitude limiter excited by currents modulated in frequency in accordance with signals from said one signaling condition to another signaling condition, signal current amplifying and detecting means coupling said current amplitude limiter to said locking circuit for controlling the same to make said one device conductive in said one signaling condition, and a second coupling between said limiter and said locking circuit for making said one device conductive in the absence of said modulated currents at the current amplitude limiter.

3. In a frequency shift telegraphy system including a locking circuit having one electron control device which is non-conductive in signal marking condition, a current amplitude limiter having an input excited by high frequency currents shifted in frequency between one frequency representing mark and another frequency representing space, an oscillationgenerator having an output circuit tuned to a frequency integrally related to themean frequency of said frequency shifted currents, means for entraining said generator by the output of saidcurrent amplitude limiter, a discriminator and detector having its input excited by the output of said generator, said detector supplying as output a direct current component which varies between two values, one representing mark and the other space, a coupling between said detector output and said locking circuit such that said one device is rendered non-conductive by appearance of a direct current potential in the output of said detector representing mark, and a coupling between the current amplitude limiter vand said locking circuit such that said one device is non-conductive in the `absence of input to said current amplitude limiter.

4. In a. frequency shift telegraphy system including a two-electron-control device locking circuit having one device which is biased to cutoff in signal marking condition, a current amplitude limiter electrode structure having an input excited by high frequency currents shifted in f-requency between one frequency representing mar and Vanother frequency representing space, said limiter structure having electrodes in a biasing circuit wherein is developed a potential of -a magnitude depending on the strength of the high frequency currents, an oscillation generator having an output circuit tuned to the mean frequency of said frequency shifted currents, means for entraining said generator by the output of said current amplitude limiter, a discriminator and detector having its input excited by the output of said generator, said detector having as output a direct current component which varies between two values, one representing mar and the other space, a coupling between said detector output and said locking circuit such that said one device is rendered nonconductive by appearance of a direct current potential in the output of said detector representing marking condition, and a coupling electron control device between said biasing circuit of said current amplitude limiter and said locking circuit for biasing said one device thereof to cutoff in the absence of input to said current amplitude limiter.

5. In a telegraphy system in combination, signal recording apparatus including a locking circuit having two electrode structures with their electrodes so coupled that when current flows in one structure current is cut off in the other structure and vice versa, a current amplitude limiter electrode structure having input electrodes excited by telegraphy currents keyed between two conditions, one representing mark and the other -representing space, said current amplitude limiter having in a biasing circuit therefor an impedance the potential drop in which depends upon the magnitude of the telegraphy currents, detecting means having an input coupled to said current amplitude limiter output, a coupling between the output of said detecting means and the control electrode of a structure in said locking cir-cuit for tripping the same to one condition of stability in the presence of telegraphy currents representing mark, and a coupling electron control device between the impedance in said limiter biasing circuit and an electrode in one of said structures in said locking circuit to trip the same to said one condition of stability in the absence of telegraphy signals at the input of said limiter.

J. C. E. MITCHELL.

(References on following page) REFERENCES CITED The following references are of record in the v111e of` this patent:

Number UNITED STATES PATENTS Name Date Hex-'man Mar. 10, 1931 Taylor June 30, 1936 Cannon Oct. 1, 1940 Peterson et al July 29, 1941 Hudc Aug. 11, 1942 Hansen Aug. 18, 1942 Weagant Dec, 15, 1942 Number 16 Name Date Cox Dec. 291942 Trevor June 13, 1944 Beers Aug. 22, 1944 Earp Sept. 19, 1944 Koch July 3, 1945 Hollingsworth Aug. 21, 1945 Hansell Oct. 30, 1945 BaileyA Apr. 2, 1946 Rhodes Apr. 2, 1946 Sands Nov. 12, 1946 Bond Feb. 18, 1947 Corrington Apr. 27, 1948 

